If a Mastodon instance was run by someone who allowed a genocide to be fuelled by their platform, and earn money from the advertising, I think we'd defederate in a heart beat. It just doesn't seem consistent to federate with them.
This post doesn't address the main "pro-federation" point that I have seen. People who are support of federation aren't saying that Facebook is a great company, they have great morales or that they aren't supporting ActivityPub for their own gain. I think there is very little doubt that FB is a shit organization with no morales who thinks that this is a great move to get people back to their sites.
The most common reason that I see people supportive of Threads' federation is that they believe it will help people move off of Facebook and other proprietary platforms onto more user-friendly ones. If all of your friends use Instagram it is very hard to move to Mastodon. If you want to stay in touch you will at least need two accounts. You can move friends but it is hard because they each need to make that switch and it affects their interaction with others, or they need to manage multiple accounts until most of their friends have switched. If your friends use Threads (and it federates) then you can switch to Mastodon with very little friction, you can still interact with all of your existing friends in more or less the same way. Similarly each friend can easily move without managing multiple accounts during the transition. If all instances have blocked threads.net many people just won't move, they will stay with FB.
To make a good argument you need to either refute this perceived advantage or argue that it isn't worth the downsides. Making up a strawman doesn't convince anyone.
I get what you're saying, I think most people fear that instead FB's septic tank will spill and spread in the Fediverse instead; I already had to deal with some "FREE SPEECH!" guy that wanted it to be a platform where people for example from Hamas and Israel could discuss and "agree to disagree".
I'm not at my computer, so please excuse any mobile issues. I'm in favor of the move, because it will help to simultaneously connect and decentralize communications across the platforms. Say what you will about Facebook (you're probably right), but if they're that bad, then it seems logical to me to connect to their federated service even more aggressively.
The more we push our content (and by extension the Fediverse content that kbin aggregates), the less impact their algorithm can have. The more we go out of the way to expose their content, the harder we make it them to curate/censor/suppress any voices. And if, when comparing two Fediverse instances or softwares, we find that what's been pushed to them is different, we the users can call it out to news organizations (or make it public ourselves).
And yes, I know I'm making the arguments for supporting private companies in adopting open-source. It's about being able to audit what companies we don't trust are doing.
In addition to that, I'm currently a Threads user. Anecdotally, there's a lot of wholesome content on there that I appreciate, and what limited advertising is there is nowhere near as obtrusive as Reddit or the main Facebook platform.